A known clutch arrangement includes friction plates enclosed in a clutch housing. The friction plates are compressed mechanically or pneumatically by a circular piston pressing on a pressure plate, which comes into contact with the friction plates and pushes them together. An output shaft having the load to be driven attached to it, e.g. by a coupling, has alternate friction plates in mechanical cooperation with it. The other alternate friction plates (i.e. those not in cooperation with the output shaft) are in mechanical cooperation with a driving sleeve which rotates as part of the driving shaft. The clutch works by friction acting between the friction plates as they are pushed together.
This type of clutch relies on the constancy of e.g. operating fluid pressure and/or the load to be driven. In other words, the important factors affecting the effective operation this type of clutch are how much force can be applied by the piston to push the friction plates together, and how much load is attached to the output shaft to be driven. Obviously, a larger load has more inertia and is harder to start rotating. Larger forces, e.g. larger fluid pressures, are needed in this case. If demand on the clutch is raised beyond its capacity, the friction plates are likely to slip and overheat; the clutch will eventually fail by burning out.
Another example of a known clutch is shown in GB 2216203. The arrangement shown in this document has an internally splined driving sleeve movable under the action of a pneumatic ram to engage an output dog drivably connected to an output shaft—this engagement effectively makes the driving sleeve and output shaft a single mechanical member, thereby avoiding the dependence on operating air pressure. The ram has an actuating rod with a fork element attached to it, the fingers of the fork element engaging an annular groove in the outer surface of the driving sleeve. Thus, when compressed air acts on an end of the pneumatic ram, the actuating rod slides axially, moving the driving sleeve with it. The driving sleeve has a pressure plate located inside it and releasably engaged to it by means of steel balls resiliently urged into depressions formed in the inner surface of the sleeve. There are a set of friction plates, alternate ones of which are engaged with the internal splines of the driving sleeve, the remainder being engaged with the output shaft. When the sleeve is initially moved towards engagement with the output dog, the pressure plate moves axially with it and loads the friction plates against one another to begin turning the output shaft. A large torque is required to start the rotation because of the inertia of the load attached to the output shaft. This torque manifests itself as friction between the friction plates and the internal splines of the driving sleeve. This friction is enough to prevent further sliding motion of the driving sleeve until the rotational speeds of the driving sleeve and output shaft are more or less equal. The torque required to turn the output shaft is then less, so the friction acting on the splines of the driving sleeve is reduced and sliding recommences.
In this arrangement, the force that pushes the friction plates together originates from the compressed air acting on the pneumatic ram. Since the size of the ram is limited e.g. by the constraints of the size of the housing itself, the amount of force that can be generated is also limited. Furthermore, when large loads need to be started, the bending moment on the fork element deflects the line-ability of the clutch housing. As a result, the pneumatic ram is pulled out of alignment with the housing, which can wear the components. As more powerful machinery with larger loads that need to be driven is introduced, clutches of this type are struggling feasibly to provide enough force to overcome the initial inertia so as to enable engagement.